Introduction

Figure 1: Schematic of the SCNT Technique

Somatic Cell Nuclear Transfer (SCNT) is a technique, which allows the harvesting of embryonic stem cells (ES cells). ES cells are pluripotent, because they can undergo continual self-renewal without becoming differentiated like other body (somatic) cells.[1] These can then be used as treatment in human and veterinary medicine such as patient-tailored tissue grafts, as well as allowing the possibility of cloning entire organisms such as Dolly the Sheep. SCNT avoids or at least, minimises the potential ethical issues that other embryo sources, such as surplus in-vitro fertilisation embryos, have encountered in many countries including Australia (see Stem Cell Controversy).

SCNT involves taking any differentiated somatic cell's nucleus, for example, skin and muscle cells, and placing that nucleus into an unfertilised egg cell. The egg's own nucleus has already been removed (see Figure 1). This initiates a process called nuclear reprogramming, which causes the donor nucleus to become pluripotent again by certain chemicals in the egg's cytoplasm.[2] The combined hybrid egg can develop into an embryo without normal fertilisation, and the EScells can be extracted.

Stem Cells in General

As noted before, ES cells are pluripotent, which means they are the first cells to form in embryoes. Subsequently, they will undergo continual division to become all the cell types in our bodies. Their pluripotent property is similar to cancer cells, however the stem cell process is tightly controlled, unlike the random growth of tumours[3].

Stem cells can be found in two places in a human:

(1) In a developing embryo called a blastocyst (see Figure 2) - only the inner cell mass (ICM) contains stem cells. The surrounding shell of trophoblast cells do not - they give rise to the placenta, instead.

(2) Adult and child tissues such as bone marrow and skin contain stem cells, which produce new cells to regularly replace old/dead cells. However they are rarer and difficult to identify amongst the crowded somatic cells, and do not survive long enough outside the human body for potential therapeutic use.

Timeline for Stem Cell & SCNT Research

1981 Martin Evans at the University of Cambridge[4] and Gail Martin at the University of California - San Francisco[5] successfully cultured pluripotent mouse ES cells from ICMs of late blastocysts derived from an established teratocarcinoma stem cell line.

1995 James Thomson and his colleagues at the University of Wisconsin[6] isolate the first embryonic stem cells from rhesus macaque monkeys. The research shows it is possible to derive embryonic stem cells from primates, including humans.

1997 Ian Wilmut and colleagues announced the creation of the first cloned mammal using SCNT - a baby lamb that was subsequently named Dolly.[7] Dolly the Sheep was groundbreaking and highly controversial, raising debates amongst laymen, theologians and government officials on the seemingly inevitable cloning of human beings.

1998 Two research groups independently announce that they have derived stem cell lines from embryonic tissue, however their research is controversial due to the sources of the used embryos:

James Thomson's group at the University of Wisconsin[8] used ICMs from in-vitro fertilization clinic surpluses.

Michael Shamblott's group at the Johns Hopkins University School of Medicine[9] used primordial germ cells from aborted fetuses.

Jose Cibelli and Michael West publicize their experiments. [10] The experiment was conducted at the University of Massachusetts and allowed the university to receive a patent in 1999 for the technology.

2001 US President George W. Bush authorises selected ES cell lines to be used for human research, however other cell lines are not allowed. Most of these cell lines are subsequently found to be non-viable and of no use.[11]

2004 South Korean researchers, Hwang Woo-Suk and Moon Shin-Yong, publish that they are the first to clone a human embryo using SCNT from a female cumulus cell and have it reach the 100-150 cell stage.[12] Viable clones were only obtained when the donor and recipient was the same person. Cells propagated for one year and could develop into cartilage, bone, and muscle when implanted in a severely immunodeficient (SCID) mouse.

2005 Two research teams publish reports online that attempt to address the loss of life issue associated with the derivation of ESCs:

Robert Lanza’s group established ES cell lines using only one cell from an eight-cell mouse blastomere, which maintained the embryo’s viability in the womb.[13]

US President George W. Bush vetoed the Stem Cell Research Enhancement Act (see H.R. 810) and approved the Fetus Farming Prohibition Act (see Public Law 109-242). This effectively banned any human stem cell research in the public sector, leaving the majority of embryonic stem cell research up to private funds.

Two research papers[15][16] by Hwang Woo-Suk (see Figure 4) and his colleagues, submitted to the journal Science in 2005, were editorially retracted for consisting largely of fabricated data.[17]

2009 The US Food and Drug Administration (FDA) approves the first human clinical trials for complete spinal injury patients to determine the safety of injecting ES cells into such patients.[18] These ES cells can be used, because they had previously been authorised in 2001

2010 The US FDA approves human clinical trials, conducted by the Medical College of Georgia, for children with cerebral palsy. These trials do not use SCNT, but rely on ES cells present in umbilical cord blood.[19]

Methodology

The actual techniques in SCNT are common between different researchers, however they differ by the materials and equipment used, for example, specific growth chemicals and electric fusion voltages. Apart from this, they all are common centrally - they involve the nucleus of a somatic cell (eg. a normal body cell such as a blood cell, heart cell/cardiocyte, skin cell/fibroblast; the sperm and egg are germ cells not somatic cells) being physically transferred into an unfertilised egg cell that has had its own nucleus removed (referred to as 'enucleation').

Stated below is an outline of the steps you would take in order to perform SCNT for humans.

Step 1: Preparation of the somatic cell

The somatic cell, as stated, can be any type of normal cell in the body apart from the sperm or egg. Most researchers appear to favour skin fibroblasts, because the skin is easy to access, non-invasive and fairly painless. However, cells from the breast/mammary gland[20] and cumulus cells[21] have also been used.

A tiny amount of skin is cut and placed in a trypsin enzyme-buffer solution that frees the target fibroblasts from the extracellular matrix. The mixture is placed on a serum medium and incubated for three weeks, in order to obtain a single layer of fibroblasts without any other cell types.

Figure 5: Step 2 - Generating cytoplasts by oocyte enucleation.

Step 2: Preparation of the egg/oocyte

Typically, researchers will select the target egg that is in the antral stage and exhibits the 1st polar body. When the researchers can see follicles at least 18mm wide, human chorionic gonadotropin (hCG) is injected into the female donor.[22] hCG is used since it is a strong inducer of ovulation, and allows a more 'comfortable' way of obtaining the egg without any invasive and direct surgical procedure to the ovaries and the donors themselves. Consequently, the ovulated egg is collected by ultrasound-guided transvaginal needle aspiration in a procedure similar to in-vitro fertilisation.

Fluorescent tags are bound to the oocyte's DNA, allowing the researchers to check all the oocyte's DNA/nucleus has been removed when exposing the egg to UV light.[24]

The egg's nucleus is removed using an inverted microscope, UV light and a glass needle. This setup minimises damage to the delicate egg as it can cut through the thick zona pellucida shell, and is fairly easy to manipulate. At this point with its nucleus removed, the oocyte is called a cytoplast (see Figure 5).

Step 3: Nuclear Transfer

Both fibroblast and egg are placed in a thin human serum solution[25] with cytochalasin B.

Once the donor fibroblast's nucleus is extracted from the fibroblast with a pipette, it is called a karyoplast. Subsequently, this karyoplast is injected into the egg/cytoplast past the zona pellucida (see Figure 6).

At this point, the karyoplast and cytoplast are still functionally separate (see Figure 7 at 0 minutes), therefore a few electric pulses are given to the entire solution causing fusion between the two entities (see Figure 7 at 10 minutes).

Step 4: Post Nuclear Transfer Procedures

The complete process of nuclear transfer is completed approximately 35-45 hours after the original hCG was administered to the female donor. However, it takes an additional three hours before cleavage can be seen if the transfer and activation has been successful.

Finally, the egg is incubated in a culture medium at 37°C in highly humidified conditions. This could be both an artificial attempt and natural requirement that replicates the uterine conditions, which are conducive to embryonic development. After this activation, in approximately four days for human donors, cleavage of the egg can be clearly seen.

It has been noted by Dominko and colleagues[26] that the oocyte can be from any mammal if using a mammalian nucleus. It does not necessarily mean a human fibroblast must require a human egg for use. This is, because the initial development of all mammal eggs undergo a similar process, and it is only later in the morphogenesis of the embryo that the nucleus's DNA actually starts taking control of the process. The consequence of using a cow egg with a human fibroblast or other species' fibroblast is the first 2 cleavages correlated to bovine development time, while after these two divisions, the growth rate and timing of the embryo matched the donor species.[27] Therefore, if human nucleus and oocyte were used, the entire process would mirror the normal human rate.

For applications in regenerative medicine (obtaining specific cell/tissue types that can be surgically grafted for a patient): the ICM is harvested from the blastocyst onto mice-derived feeder cells for nutrients (see Figure 9) and differentiated into the required tissue/cell types, using certain growth and differentiation factors over two days.[28]

The actual differentiation factors required for specific somatic cells has been determined over the years by many different researchers, for example, stem cells exposed to dimethyl sulfoxide would diffentiate into different proportions of muscle cells, while stem cells exposed to retinoic acid would become neurons[29]

Apart from the differentiation factors, the removal of the feeder cells or the cells' chemical messengers (cytokines) would also be required to signal the embryonic stem cells to differentiate.[30][31]

Current Situation

Unfortunately, current research has not been successful in producing human embryoes that develop the blastocoele which is essential for later gastrulation and morphogenesis. It appears the cells remain alive and viable as the cell numbers keep increasing, but they fail to organise themselves into any identifiable embryonic structures. In other primates such as monkeys the production of embryonic stem cells has also been unsuccessful - apparently as a result of large amounts of genetic instability.[32]

There is a high rate of blastocysts that are transplanted into surrogate mothers that result in none reaching full term. Instead, the fallopian tubes, where the embryo was transplanted, develop large sacs of fluid surrounding the dead blastocyst.

Due to ethical issues, the transplantation of human blastocysts in reproductive cloning has not occurred to date. However, blastocyst transplants of other animals such as cows have been successful in producing undamaged and living offspring.[33]

Applications of SCNT

Veterinary, Animal Science

Mass production of animals: As farm animals are being used for human use, SCNT can be used to produce high quality farm animals in infinite number. Cloning technology can be applied, without compromising animal welfare, if integrated in breeding programs and these transgenic clones will be delivering the expected products.[34]. Researches show that, somatic cell cloned cattle reportedly were physiologically, immunologically, and behaviorally normal and this makes use of SCNT useful for mass production.[9]

Conserving wild animals for next generations: Another area where SCNT can be useful is conservation. This use can be effective to preserve and propagate endangered species that are being produced poorly in the zoos.[35] With effective reproduction, these species can be reintroduced to the wild again, allowing maintenance of genetic diversity of species by introducing new genes. The use of SCNT can also be helpful to even create the extinct species, if any tissues or cells are available.[36] The idea of producing mammoth is being considered as an intact animal was discovered frozen in the tundra. The close relative of the mammoth, the elephant, could be used both as a surrogate mother and an oocyte donor.

As also mentioned by Holt et al, [10], Reproductive cloning, by nuclear transfer, is often regarded as having potential for conserving endangered species. Cloning non-mammalian vertebrates can be more practical than using conventional reproductive methods. As cloning technology has made a good progress in amphibians, it may be possible to breed threatened amphibians and even reproduce extinct amphibian species.

Disease resistant animal production: By using SCNT, genes causing diseases can be manipulated in order to have healthier farm animals that live a lot longer.[37]

Human Medicine

Human therapeutic proteins: Human proteins are needed and in demand for the treatment of diseases. Purifying proteins from blood is an expensive procedure and also carries the risk of contamination by Hepatitis C or HIV. Proteins can be produced in human cell culture but the output is small and it is also an expensive procedure. But also, Ng et al. [38]mentions that human proteins can be produced in the milk of transgenic sheep, goats and cattle. The output can be as high as 40 g per litre of milk and the cost of the procedure is not as high. By using nuclear transfer, it is possible to insert human genes at specific points in the genome, improving the reliability of their expression; deleting, substituting and adding of genes that are missing in the patient to lead them have better lives, possible as well.

Xenotransplantation: Shortage of organs is a big problem considering the amount of patients needing them. Transplant organs can be a solution for this. Genetically modified animals such as pigs are being developed as a solution but so far the modifications are limited to adding genes. By using SCNT, deleting genes that are responsible for rejection from pigs is possible and this way it is aimed to avoid rejection of an organ transplanted from a normal pig to a human patient.[39]

Animal model for diseases: experimental animals with altered disease-causing genes can be tailor generated using SCNT, allowing better understanding of the complex pathogenesis of the diseases, eg. good SCNT-mouse models would allow cystic fibrosis between the lung and intestine to be understood better[40][41]

Cell Therapy using dedifferentiated stem cells: This use is being developed for a range of diseases including heart attack, stroke and diabetes etc. Patient’s own cell can be used as transplanted cells are likely to be rejected. Cloning of adult animals shows that egg and the embryo have the capability of reprogramming. This use may make it possible to reprogramming patient’s own cells without creating and destroying embryos. The differentiating stem cells can then be grown into several hundred or thousand cells and surgically transported into the patient where they will produce the required tissue.[42]. To give an example, a child's problem of severe immnunodefficiency due to chemotheraphy and whole body radiation because of having Hodgkin's Lymphoma, can be corrected as child's skin fibroblast can be obtained and embryonic stem cells obtained as per the procedures mentioned above. Transcription factors would differentiate the stem cells into bone marrow cells, which can be transplanted into the marrow cavities of the child, and they would gradually rebuild the child's haemopoietic system and also, their immune system.

SCNT can also be used in treatment for Cardiovascular and Nervous System Disorders. A study by Eschenhagen et al,[43], shows that tissue engineering for cardiac development and cardiac repair by SCNT is possible, that is creating contracting heart mucles from patients own stem cells and use them to decrease the intensity of the disease.

Developmental Biology

Events during fertilization and pre-implantation embryos: knowledge of the complex mechanisms and the various controlling factors during embryonic development will be understood better with SCNT.[44]

Advantages and Disadvantages of SCNT

Advantages of SCNT

Disadvantages of SCNT

Retains genetic code of the donor nucleus: Resultant tissue that is transplanted into the donor patient will not be exposed to potentially fatal immune rejection.[45]

Common Procedures and Methods: Relatively similar and uniform between different researchers - the main difference appears to be the choice of culture mediums which do not seem to affect the actual outcome.[47]

Difficulty in inducing the re-expression of differentiated genes: This is especially the case where the donor nuclei have been obtained from adults as opposed to fetal or newborn stage.[49][50] This phenomenon appears to be in common with in vitro fertilisation in human and non-human species, where higher rates of spontaneous pregnancy terminations occur with adult-age nuclei, as opposed to higher term births with fetal/embryonic nuclei.[51]

Electric Fusion damage: Eiges and colleagues[52] have suggested electroporation, which has been demonstrated to be fatal to stem cell survival in other non-SCNT techniques, may disrupt the delicate interaction between the fusing oocyte and somatic nucleus, thereby preventing them from communicating properly. Other methods for cytoplast-karyoplast fusion in SCNT have not been assessed yet.

Cross-species incompatibilities: Gurdon et al.[53] suggests irreconcilable genetic differences between egg and nucleus species leads to low success rate; that is, mice eggs should not be mixed with human nuclei. Though amphibian and mammal sources were used by Gurdon and colleagues, there has been no other evidence to suggest why this incompatibility could not affect human-only procedures as well.

Future Directions

Fulka and colleagues[54] has suggested issues that need to be solved for SCNT in order for progress to be made such as:

Cytoplast age

Cytoplast cell-cycle stage

Activation procedure: methods of activating the karyoplast-cytoplast so that the embryo is not harmed (as noted in the Disadvantages) need to be found.

Source of karyoplasts and their degree of differentiation

Karyoplast cell cycle stage: in the case of regenerative medicine, the donor nucleus age would be very important since many patients who could potentially benefit from this therapy would be quite elderly. However, the low success of obtaining viable ICM have not allowed researchers to determine the effects of age on the donor nucleus, if any.[55]

Karyoplast-cytoplast (nuclear-cytoplasmic) interactions

Species-specific differences

Technical aspects: for example, the currently used feeder fibroblast cells are derived from mice, which introduces the possibility and already reported occurrences of cross-contamination of human embryonic cells by mouse disease-causing pathogens.[56] This contamination issue can be addressed by using screened human fibroblast feeder cells or eliminate feeder cells,[57] and use serum-free medium only, especially if the cultured cells are for therapeutic use.[58]

According to Trounson[59], the questions of whether the cells undergo differentiation or transdifferentiation during offspring development and also how these changes are controlled are sources of ongoing debates. Regardless of the outcome of this debate, using SCNT has a place in future in research. In the future the uses of SCNT could be:

production of transgenic mice

rapid production of genetically modified herds or elite livestock individuals with desirable traits in agriculture

production of patient-specific embryonic stem cells in human medicine

In recognition of the stalled progress in SCNT, Maherali et al.[60]suggested the use of four transcription factors (c-Myc, Sox2, Oct4 and Klf4) can cause somatic cells to revert into embryonic-like stem cells, referred to as induced pluripotent stem cells (iPS cells), without needing the entire SCNT procedure. However, Okita and colleagues[61] expressed concern such cells display an increased likelihood to develop into cancerous cells as a result of both using c-Myc factor and reactivating the c-Myc transgene. As a result, Yu et al.[62] have indicated the use of a different set of transcription factors (Oct4, Sox2, Nanog and Lin28) are both sufficient and without the side-effects of c-Myc to obtain the iPS cells. In 2007, Byrne et al.[63] successfully obtained sustainable iPS cell lines from the rhesus macaque monkeys. This has so far not been accomplished by SCNT in human or other primates.

It is plainly evident that researchers require more progress into the field of SCNT for humans. There is still much to be learnt and understood about the actual mechanisms that occur during the fusion of the oocyte and somatic cell.[64] Hall and Stojkovic [65] theorise, though the future will bring more improvements and uses for SCNT, human SCNT may remain clouded in ethical, moral, and religious controversies.

Ethics

stduent drawn diagram of Ethics & SCNT

SCNT is considered a new technology. Due to being new, all uses of this technology have not been explored yet and it is not certain yet which uses of this technology can be acceptable or not. Safety and also inefficiency of the procedure is another concern as relatively few births have resulted from many attempts in cloning technology and this has brought some ethical and moral concerns. Morally, seeing scientists in the role of ‘God’ have been a question in minds as this idea, cloning a human, created a big debate amongst lawmakers, religious leaders, academicians and professional societies. The idea of SCNT, has violated moral values and traditions and this raised question marks which contributed to the passage of restrictive laws in several nations and to proposals for restrictive legislation in USA.

A positive idea defending the use of reproductive SCNT is that the cloning technology can give a positive progress in medicine and other fields to improve quality and conditions of life as SCNT can be used for therapeutic purposes and produce embryonic stem cells for people who need organ and tissue transplants and also in infertile couples provided that the safety of procedure can be guaranteed.

The Religous and Legal Issues:

World religions have different approaches to stem cell and hES research. Here are perspectives of major religions according to Lori P.Knowles, [11] :

Greek Orthodox and Roman Catholic Churches: Authorities of Greek Orthodox and Roman Catholic have come in favor of stem cell research using adult stem cells but they have opposed hES research as illegal and immoral as they see it as human person begins at conception and the human embryo has the same moral status as human persons. So, the reserach on human embryos, including hES is seen as willful destruction and is homicide.

Judaism: Unlike Greek Orthodox and Roman Catholic Churches, The rules of the Jewish culture do not see use of hES as immoral as they say the fetus does not become a person until the head emerges from the womb.

Islam: Islamic belief is against human suffering and illness, means that the use of surplus IVF embryos for stem cell research
is relatively uncontroversial. But creating embryos for the purposes of research is still argued.

Legal Issues

With Prohibi-tion of Human Reproductive Cloning and the Regulation of Human Research Amendment Bill 2006 through parliament, Australia has joined the nations that fund and regulate Somatic Cell Nuclear Transfer research, [12]

SCNT Research in Australia

Funding was granted by NSW and Victorian Governments to two research centers in 2008, to encourage research in development of use of SCNT. One of them is research center based at Fertility East in Sydney collaborating with the Monash Institute of Medical Research and the other one is Sydney IVF collaborating with the Australian Stem Cell Center. Outcomes of the research studies are expected soon.

Despite the ambiguity of SCNT research for human use, SCNT research in other animals has been thriving in Australia - for example, Lee and his colleagues at the University of New South Wales were able to demonstrate that enriched stem cells placed in chemotherapy-treated muscle tissue were able to regenerate more optimally than without the enriched stem cells.[66] Though the experiments were performed in mice and also have not been licensed for any human clinical trials, the study allows us to understand possible mechanisms and properties that will be encountered in regenerative medicine - a large applicative field of SCNT.

Government's funding will be supporting Victorian stem cell reserachers collaborating with Californian scientists. The funding is focusing on research into Stem Cell Transplantation Immunology which is aimed at ensuring human immune tolerance of stem cell derived cell and tissues, as skin, bone and organ tissues. [13]

Links to Research Laboratories and Researchers

The International Society for Stem Cell Research (ISSCR) is an independent, nonprofit organization formed in 2002 to foster the exchange of information on stem cell research. Learn more about this exciting new organization.

Fertility East[15] is a new independent Fertility and IVF unit started in June 2007 collaborationg with Monash Institute of Medical Research[16]and both granted by the government in 2008 in reasearch of SCNT.

Sydney IVF[17] and Australian Stem Cell Center[18] are collaborating together in development of use of SCNT and granted by the government in 2008.

Glossary

Antral Stage: a very mature oocyte-containing follice in the female ovary; usually ready to undergo ovulation.

Blastomere: is a type of cell produced by division of the egg after fertilization.

Blastocyst: A thin-walled hollow structure in early embryo development that contains a group of cells called the inner cell mass from which the embryo arises.

Cleavage: The act or state of splitting or dividing of a cell, particularly during the telophase of (animal) cell division.

Cumulus cell: cumulus oophorus granulosa cells, cumulus oophorus granulosa cells, At one part of the mature follicle, the cells of the membrana granulosa are collected into a mass which projects into the cavity of the follicle.

Cytochalasin B:fungal metabolites that have the ability to bind to actin filaments and block polymerization and the elongation of actin; permeate cell membranes, prevent cellular translocation and cause cells to enucleate.cytochalasin A and cytochalasin B can also inhibit the transport of monosaccharides across the cell membrane

Cytoplast: the inner part of the cell without cell wall and plasma membrane. It includes cytroskeleton, organelles and cytosol.

Cytokines: are any of a number of small proteins that are secreted by specific cells of the immune system and that carry signals locally between cells, and thus have an effect on other.

Dolly: the first mammal cloned from differentiated cells

Fetal cell serum: Similar to fetal bovine serum[22] since it is essentially blood with all cells/platelets have been removed, however can be obtained by coagulating donated human blood. Both human and bovine serum can be used in SCNT with no side effects.

Human chorionic gonadotropin: produced by the female to sustain the early stages of pregnancy; to stimulate ovulation.

Karyoplast:A cell nucleus surrounded by a narrow band of cytoplasm and a plasma membrane. (Retrieved from [23])

Luteinising Hormone: A hormone produced by the anterior lobe of the pituitary gland that stimulates ovulation and the development of the corpus luteum in the female and the production of testosterone by the interstitial cells of the testis in the male. (Retrieved from [24])

Plasma estradiol concentrations: represents the major estrogen in humans; Estradiol has not only a critical impact on reproductive and sexual functioning, but also affects other organs including the bones.

Somatic cell:any cells forming the body of an organism, as opposed to germline cells.

Transvaginal needle aspiration: or oocyte retrieval (OCR) is a technique used in in vitro fertilization in order to remove oocytes from the ovary of the female, enabling fertilization outside the body. It is commonly known as "egg collection."

Trophoblast: The outermost cell layer of the blastocyst that attaches the fertilized ovum to the uterine wall and conducts nutrients from mother to developing child. (Retrieved from [25])

Trypsin: a natural enzyme that can be found in the human gut; breaks down proteins/peptides; used experimentally to inactivate any protein/enzymes/bacteria in the medium.

Xenotransplantation : The surgical transfer of cells, tissues, or especially whole organs from one species to another, such as from pigs to humans. (Retrieved from [26])